Semiconductor devices with germanium-rich active layers &amp; doped transition layers

ABSTRACT

Semiconductor device stacks and devices made there from having Ge-rich device layers. A Ge-rich device layer is disposed above a substrate, with a p-type doped Ge etch suppression layer (e.g., p-type SiGe) disposed there between to suppress etch of the Ge-rich device layer during removal of a sacrificial semiconductor layer richer in Si than the device layer. Rates of dissolution of Ge in wet etchants, such as aqueous hydroxide chemistries, may be dramatically decreased with the introduction of a buried p-type doped semiconductor layer into a semiconductor film stack, improving selectivity of etchant to the Ge-rich device layers.

This is a Continuation application of application Ser. No. 14/301,281filed Jun. 10, 2014, which is a Continuation application of applicationSer. No. 13/717,282 filed on Dec. 12, 2012.

TECHNICAL FIELD

Embodiments of the invention are in the field of semiconductor devicesand, in particular, semiconductor devices having germanium (Ge) activelayers.

BACKGROUND

For the past several decades, the scaling of features in integratedcircuits has enabled increased densities of functional units on asemiconductor chip. For example, shrinking transistor size allows forthe incorporation of an increased number of memory devices on a chip,leading to the fabrication of products with increased capacity.

In the manufacture of field effect transistors (FETs) for integratedcircuit devices, semiconducting crystalline materials other than siliconmay be advantageous. An example of one such material is Ge, which offersa number of potentially advantageous features relative to silicon, suchas, but not limited to, high charge carrier (hole) mobility, band gapoffset, a different lattice constant, and the ability to alloy withsilicon to form semiconducting binary alloys of SiGe.

One problem with the use of Ge in modern transistor designs is that theextremely fine features (e.g., 22 nm and below) that are now achievedfor silicon FETs aggressively scaled over the years are now difficult toachieve in Ge, often making potential material-based performance gains awash when implemented in less-aggressively scaled forms. The difficultyin scaling is related to the material properties of Ge, and moreparticularly difficulty in etching SiGe, which is often employed as anintermediate layer between a Ge active layer (e.g., transistor channellayer) and an underlying silicon substrate material, with sufficientselectively over Ge so as to remove the SiGe without eroding a finelyprinted Ge active layer feature.

Material stack architectures and etching techniques which enable highSiGe:Ge etch selectively are therefore advantageous.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a cross-sectional view of a semiconductor layerstack including a germanium device layer disposed over a delta-dopedp-type transition layer, in accordance with an embodiment of the presentinvention;

FIG. 1B illustrates a plot of dopant concentration depth profile of asemiconductor layer stack including a delta-doped p-type transitionlayer, in accordance with an embodiment of the present invention;

FIGS. 2A and 2B illustrate cross-sectional views of a local growth ofthe semiconductor layer stack depicted in FIG. 2A, in accordance with anembodiment of the present invention;

FIGS. 3A and 3B illustrate cross-sectional views in the fabrication of aplanar semiconductor device employing the semiconductor stack of FIG.1A, in accordance with another embodiment of the present invention;

FIGS. 4A-4C illustrate angled views representing various operations in amethod of fabricating non-planar semiconductor devices employing thesemiconductor stack of FIG. 1A, in accordance with embodiments of thepresent invention;

FIG. 5A illustrates an isometric sectional view of a nanowire ornanoribbon semiconductor device employing the semiconductor stack ofFIG. 1A, in accordance with an embodiment of the present invention;

FIG. 5B illustrates a cross-sectional channel view of the nanowire-basedsemiconductor structure of FIG. 5A, in accordance with an embodiment ofthe present invention;

FIG. 5C illustrates a cross-sectional view of the nanowire-basedsemiconductor structure of FIG. 5A, in accordance with an embodiment ofthe present invention;

FIGS. 6A-6D illustrate isometric sectional views representing variousoperations in a method of fabricating a nanowire semiconductor devicehaving, at least at one point in the process, a germanium device layerdisposed over a p-doped transition layer, in accordance with anembodiment of the present invention; and

FIG. 7 illustrates a computing device in accordance with oneimplementation of the invention.

DETAILED DESCRIPTION

Semiconductor devices having Ge-rich active layers disposed over a dopedsemiconductor transition layers are described. In the followingdescription, numerous details are set forth. It will be apparent,however, to one skilled in the art, that the present invention may bepracticed without these specific details. In some instances, well-knownmethods and devices are shown in block diagram form, rather than indetail, to avoid obscuring the present invention. Reference throughoutthis specification to “an embodiment” means that a particular feature,structure, function, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention.Thus, the appearances of the phrase “in an embodiment” in various placesthroughout this specification are not necessarily referring to the sameembodiment of the invention. Furthermore, the particular features,structures, functions, or characteristics may be combined in anysuitable manner in one or more embodiments. For example, a firstembodiment may be combined with a second embodiment anywhere the twoembodiments are not mutually exclusive.

The terms “coupled” and “connected,” along with their derivatives, maybe used herein to describe structural relationships between components.It should be understood that these terms are not intended as synonymsfor each other. Rather, in particular embodiments, “connected” may beused to indicate that two or more elements are in direct physical orelectrical contact with each other. “Coupled” my be used to indicatedthat two or more elements are in either direct or indirect (with otherintervening elements between them) physical or electrical contact witheach other, and/or that the two or more elements co-operate or interactwith each other (e.g., as in a cause an effect relationship).

The terms “over,” “under,” “between,” and “on” as used herein refer to arelative position of one material layer or component with respect toother layers or components. For example, one layer disposed over (above)or under another layer may be directly in contact with the other layeror may have one or more intervening layers. Moreover, one layer disposedbetween two layers may be directly in contact with the two layers or mayhave one or more intervening layers. In contrast, a first layer “on” asecond layer is in direct contact with that second layer. Similarly,unless explicitly stated otherwise, one feature disposed between twofeatures may be in direct contact with the adjacent features or may haveone or more intervening features.

One or more embodiments described herein employ germanium-on-silicon(Ge-on-Si) substrate device architectures which further employ atransition layer, disposed between a Ge-rich device layer and a Sisubstrate, that is doped to improve the resistance of the Ge-rich devicelayer to etchants employed to remove other semiconductor layers of adevice stack composed of relatively less Ge than the device layer.

In embodiments a p-type doped semiconductor transition layer is disposedbetween a Ge-rich device layer and a Si substrate. Such arrangements maybe utilized in the formation of germanium-based transistors as planardevices, fin or tri-gate based devices, and gate-all-around devices(e.g., nanowire devices). More specifically, one or more embodiments aredirected to performing a release of rectangular-shaped Ge-containingnanowires or nanoribbons from Ge/SiGe, Ge/Si, SiGe/SiGe, or SiGe/Simultilayer stacks.

One or more embodiments described herein take advantage of a p-typeδ-doped buried semiconductor layer to enhance resistance of an overlyingGe-rich device layer to certain wet etchants useful for removing othermaterials from the semiconductor device stack, such as one or more SiGe(or pure Si) layers having relatively lower Ge content (i.e., richer inSi than the device layer), thereby improving the etch processselectivity toward a device layer of either pure Ge, or of a SiGe richerin Ge. In embodiments, the presence of the p-type doped buried layer hasbeen found to improve a Ge-rich device layer's resistance to wetetchants of SiGe employed during Ge device layer undercut and/or releaseprocesses (e.g., for gate-all-around or nanowire/nanoribbon devices),thereby conserving fine Ge-rich nanowire geometries.

The inventors have found that for certain wet etchants that aresensitive to the oxidation state of surface atoms in an exposed Ge layer(or SiGe layer relatively richer in Ge), dissolution of Ge may bedramatically decreased with the introduction of a buried p-type dopedsemiconductor layer into a semiconductor film stack. Although not boundby theory, the improved etch resistance of the Ge-rich device layer(s)is currently attributed, at least in part, to galvanic coupling betweenthe Ge-rich device layer and the buried p-type doped layer with thecharges and electronic states within the Ge-rich device layer beingmodulated by those in the p-type doped buried layer, thereby alteringgalvanic processes affecting the dissolution of Ge. Where the materiallayers between the Ge-rich device layer exposed to the etchant and theburied p-type doped layer are undoped (i.e., intrinsically dopingconcentration), the p-type doped layer can be set back below the devicelayers (e.g., 50-100 nm, or more), and still suppress etch of theoverlying Ge-rich semiconductor device layer when exposed to a wetetchant of SiGe, for example.

In one or more embodiments, the p-type δ-doped buried layer is disposedabove an n-type sub-channel leakage suppression layer of thesemiconductor device stack, which may also be a δ-doped layer. Where thep-type doped layer is disposed over an n-type doped leakage suppressionlayer, the slabs of doped material may form a doping dipole. Rectifyingcharacteristics associated with conduction band discontinuitiesresulting from the doping dipole may also play a role in observed Geetch suppression. With the material layers between the Ge-rich devicelayer and the buried p-type doped layer being undoped (e.g. intrinsic),Ge etch suppression can also be achieved with a δ-doped p-type dopedlayer having a dopant concentration that ensures mobile charge is fullydepleted by the underlying n-type doped leakage suppression layer sothat the presence of the p-type doped layer does not deleteriouslyincrease sub-channel leakage between a source and drain of a FET device.In embodiments, a p-type δ-doped buried layer may undergomigration/diffusion and spread to more than 15 nm during thermalprocessing (e.g., subsequent to an etching of SiGe selectively over Ge),but nevertheless does not fully compensate n-type dopant in the leakagesuppression layer, enabling both Ge etch suppression during fabricationand suppression of leakage in the completed FET device.

FIG. 1A illustrates a cross-sectional view of a semiconductor layerstack 100 including a Ge device layer disposed over a delta-dopedtransition layer, in accordance with an embodiment of the presentinvention. As shown, the semiconductor device stack 100 includes agermanium (Ge)-based device layer stack 108 (such as a compressivelystressed germanium layer) grown above a silicon (Si) substrate 104(e.g., as a portion of a silicon wafer).

The substrate 104 may be composed of any semiconductor material that canwithstand a manufacturing and serve as a seeding layer for crystallinegrowth of the semiconductor layers in the stack 100. In an embodiment,the substrate 104 is a bulk substrate, such as a P-type siliconsubstrate as is commonly used in the semiconductor industry. In anembodiment, substrate 104 is composed of a crystalline silicon,silicon/germanium or germanium layer doped with a charge carrier, suchas but not limited to phosphorus, arsenic, boron or a combinationthereof. In one embodiment, the concentration of silicon atoms insubstrate 104 is greater than 97% or, alternatively, the concentrationof dopant atoms is less than 1%. In another embodiment, substrate 104 iscomposed of an epitaxial layer grown atop a distinct crystallinesubstrate, e.g. a silicon epitaxial layer grown atop a boron-doped bulksilicon mono-crystalline substrate. Substrate 104 may also include aninsulating layer disposed in between a bulk crystal substrate and anepitaxial layer to form, for example, a silicon-on-insulator substrate.In an embodiment, the insulating layer is composed of a material suchas, but not limited to, silicon dioxide, silicon nitride, siliconoxy-nitride or a high-k dielectric layer. Substrate 104 mayalternatively be composed of a group III-V material. In an embodiment,substrate 104 is composed of a III-V material such as, but not limitedto, gallium nitride, gallium phosphide, gallium arsenide, indiumphosphide, indium antimonide, indium gallium arsenide, aluminum galliumarsenide, indium gallium phosphide, or a combination thereof. In anotherembodiment, substrate 104 is composed of a III-V material andcharge-carrier dopant impurity atoms such as, but not limited to,carbon, silicon, germanium, oxygen, sulfur, selenium or tellurium.

The Ge-rich device layer stack 108 may include one or more Ge devicelayers, with only a first Ge-rich device layer 108A illustrated in FIG.1A. In an exemplary embodiment, the thickness of the Ge-rich devicelayer 108A is in the range of 5-15 nm and is composed of essentiallypure Ge (i.e., some intrinsic level impurities may be present). Disposedbetween a Si substrate 104 and the Ge-rich device layer stack 108 is asilicon germanium (SiGe) buffer layer stack 106 (e.g., further includinga first layer 106A of approximately 0.5-1 μm of Si_(0.7)Ge_(0.3), and asecond layer 106B composed of approximately 0.3-1 μm ofSi_(0.3)Ge_(0.7).) to accommodate thermal and/or lattice mismatchbetween Ge and Si. Alternatively, buffer layer stack 106 may compriseSiGe having a graded Ge composition (e.g., from 30% to 70%), or multiplelayers of SiGe with varying Ge concentration, or any combinations ofthese various types buffer layer structures. In the exemplaryembodiment, the buffer layer stack 106 is disposed immediately over, ordirectly on, the Si substrate 204 with a transition layer stack 107 thatis further disposed immediately above, or on, the buffer layer stack106, and also between the Si substrate 104 and the device layer stack108.

The transition layer stack 107 includes an n-type doped SiGe layer 107A(e.g., a layer of relaxed phosphorous doped Si_(0.3)Ge_(0.7)). In theexemplary, embodiment, the n-type doped SiGe layer 107A has a thicknessof 5-20 nm with a dopant concentration in the range of 1e17-1e19atoms/cm³, and advantageously at least 1e18 cm⁻³. Because phosphorousand other n-type dopants, such as arsenic, are fast diffusing in bothSiGe and Ge, the n-type doped SiGe layer 107A is set back from the Gedevice layer stack 108 to reduce entrance of N-type dopants into the Gedevice layer stack 108. For example, the n-type doped SiGe layer 107Amay be 25-100 nm below the Ge device layer 108, for example separated bya semiconductor layer 107C composed of relaxed intrinsicSi_(0.3)Ge_(0.7)). Alternatively, to further improve short channeleffects, and/or leakage while the device layer 108 is in an “off” ornon-conducting state, the semiconductor layer 107C may be (or furtherinclude in addition to a thickness of intrinsic Si_(0.3)Ge_(0.7)) anundoped Si or SiGe layer of relatively low concentration germanium(e.g., <7% Ge) as an enhanced diffusion barrier. Total thickness of thesemiconductor layer 107A may therefore vary considerably.

The transition layer stack 107 further includes a p-type doped SiGelayer 107B (e.g., a layer of relaxed Si_(0.3)Ge_(0.7)). In the exemplaryembodiment, the p-type dope SiGe layer 107B is a δ-doped layerapproximating a 2-D slab of sheet charge. In such embodiments, thep-type doped SiGe layer 107B has a thickness of 5-15 nm, achievablethrough in-situ doping during epitaxial growth of the transition layerstack 107. Greater thicknesses may also be possible, constrained howeverso as to not completely compensate the n-type doped layer 107A. In theexemplary embodiment, the p-type doped SiGe layer 107B has a dopingbetween 5e17 and 1e19 cm⁻³, advantageously at least 1e18 cm⁻³. Thep-type dopant species is boron in the exemplary embodiment, though otherp-type dopant species may be expected to perform similarly.

FIG. 1B illustrates a plot of dopant concentration depth profile of asemiconductor layer stack including a δ-doped p-type SiGe transitionlayer, such as layer 107B, disposed over an n-type doped leakagesuppression layer, such a SiGe layer 107A, in accordance with anembodiment of the present invention. The dopant concentration depthprofile illustrated represents an “as-grown” state of the semiconductorstack as opposed to an “as-annealed” state. As shown in FIG. 1B, aboron-doped SiGe transition layer has a boron concentration exceeding2e18 cm⁻³ and approximating a δ-doping of at least 1e18 cm⁻³ over anapproximate 15 nm span of depths demarked as “107B.” A phosphorus dopingreaching approximately 1e18 cm⁻³ spans the depths demarked “107A”corresponds to the SiGe transition layer n-type doped leakagesuppression layer. As shown in FIG. 1B, the phosphorus doped layer 107Ahas a greater thickness than the boron doped layer 107B and is moregraduated than the boron doped layer 107B (i.e., not δ-doped).

In embodiments, a p-type SiGe transition layer is spaced apart from anunderlying n-type SiGe transition layer by a non-intentionally doped(e.g., intrinsically doped) SiGe layer. Such a spacer layer is denoted107A′ in FIG. 1A and is of a minimal thickness (e.g., 2-5 nm) dependenton growth rate kinetics and the rapidity at which a growth chamberswitches between an n-type and p-type dopant. The spacer layer 107A′ isSiGe (e.g., Si_(0.3)Ge_(0.7)) which is grown after termination of n-typedopant and before p-type dopant is introduced. The effective doping ofthe spacer layer 107A′ is illustrated for one embodiment in FIG. 1B,where both the boron and phosphorus doping levels are below 5e17 cm⁻³.In embodiments, the spacer layer 107A′ has a thickness of 2-5 nm. Forembodiments at the upper end of this range, the layer 107A, 107A′, and107B may be characterized as a p-i-n δ-doped structure where at leastthe p-type layer is a δ-doped layer.

Depending on the embodiment, the semiconductor stack 100 may be eitherbe a “global” film stack disposed over an entire area of a substrate(e.g., substrate 104 in FIG. 1A represents an entire wafer), or a“local” film stack that is disposed over only a certain portions of asubstrate (e.g., substrate 104 in FIG. 1A represents small portion of awafer). In either embodiment, the semiconductor stack 100 may be formedwith any epitaxy technique known to be suitable for SiGe materials, suchas but not limited to CVD and molecular beam epitaxy (MBE). As employedherein, an “epitaxial” layer is in registry with the seeding surface(e.g., having a preferred crystal orientation as a result of thecrystallinity of the seeding surface). FIGS. 2A and 2B illustratecross-sectional views of one local growth embodiment where thesemiconductor layer stack depicted in FIG. 1A is grown with the benefitof aspect ratio trapping (ART).

As shown in FIG. 2A, an isolation dielectric has sidewalls 250 defininga trench 260 with a semiconductor seeding surface exposed at the trenchbottom. As shown in FIG. 2B, local and selective epitaxial growth ofcrystalline semiconductor forms a SiGe buffer layer 206B (e.g., havingproperties as described for layer 106A) over a SiGe buffer layer 206A(e.g., having properties as described for layer 106A) disposed on asubstrate 204 (e.g., having properties as described for substrate 104).Also disposed in the trench 260 are the transition layers 207A, 207B,and 207C (e.g., having properties as described for layers 107A, 207B,and 107C, respectively), and device layers 208A, and 208B withintervening sacrificial layers 209A and 209B. In the exemplaryembodiment, the sacrificial layers 209A and 209B are each SiGe layersand may each have a same composition. In advantageous embodiments, theGe concentration in the sacrificial layers 209A, 209B is lower than inthe transition layers (e.g., <70% Ge) to have a desired level of strain(e.g., 1-1.5%) relative to the transition layer 207C. In one embodiment,the device layers 208A and 208B are each essentially pure Ge. In anotherembodiment, the device layers 208A and 208B are each of a SiGecomposition that is richer in Ge than the sacrificial layers 209A, 209Bthat are either a SiGe alloy, or may be silicon.

FIGS. 3A and 3B illustrate cross-sectional views of planar semiconductordevice embodiments employing the semiconductor stack 100. Referringfirst to FIG. 3B, a semiconductor device 300 includes a gate stack 305disposed above a substrate 304. A Ge-rich device layer 308A is disposedabove the substrate 304, underneath the gate stack 305. Generally, thesemiconductor device 300 may be any semiconductor device incorporating agate, a channel region, and a pair of source/drain regions, such as, butnot limited to, a MOS-FET. In the exemplary embodiment, the device 300is a PMOS FET serving as one of complementary transistor types within aCMOS integrated circuit.

In the exemplary embodiment the Ge-rich device layer 308A is essentiallypure Ge, compressively strained by 1-2%. A SiGe transition layer 307C isdisposed above the substrate 304, below the germanium active layer 308A.An n-type junction leakage suppression layer 307A is disposed above thesubstrate 304, with the p-type Ge etch suppression layer 307B disposedbetween the transition layer 307C and the leakage suppression layer307A, as was described in the context of the stack 100. In the exemplaryPMOS embodiment, the raised source and drain regions 322 are depositedor grown heavily doped p-type (e.g., boron) and disposed above thejunction leakage suppression layer 307A, on either side of the gatestack 305. The source drain regions 322 may form p+/n junctions with then-type leakage suppression layer 307A, or not (e.g., source/drainregions 322 disposed on an upper portion of the transition layer 307C).

In the embodiment illustrated in FIG. 3A, the gate stack 305 is disposeddirectly on the Ge active layer 308A, the germanium device layer 308A isdisposed directly on the undoped SiGe transition layer 307C, thetransition layer 307C is disposed directly on the p-type transitionlayer 307B, the p-type transition layer 307B is disposed directly on thejunction leakage suppression layer 307A (with only a SiGe spacer such as107A′ there between).

The gate stack 305 may include a gate electrode 305B disposed directlyon a gate dielectric layer 305A, as shown in FIG. 3A. In an embodiment,the gate electrode 305B is composed of a metal gate and the gatedielectric layer 305A is composed of a high-K material. For example, inone embodiment, the gate dielectric layer 305A is composed of a materialsuch as, but not limited to, hafnium oxide, hafnium oxy-nitride, hafniumsilicate, lanthanum oxide, zirconium oxide, zirconium silicate, tantalumoxide, barium strontium titanate, barium titanate, strontium titanate,yttrium oxide, aluminum oxide, lead scandium tantalum oxide, lead zincniobate, or a combination thereof. Furthermore, a portion of gatedielectric layer 305A may include a layer of native oxide formed fromthe top few layers of the Ge-rich device layer 308A. In an embodiment,the gate dielectric layer 305A is comprised of a top high-k portion anda lower portion composed of an oxide of a semiconductor material. In oneembodiment, the gate dielectric layer 305A is composed of a top portionof hafnium oxide and a bottom portion of silicon dioxide or siliconoxy-nitride.

In an embodiment, the gate electrode 305B is composed of a metal layersuch as, but not limited to, metal nitrides, metal carbides, metalsilicides, metal aluminides, hafnium, zirconium, titanium, tantalum,aluminum, ruthenium, palladium, platinum, cobalt, nickel or conductivemetal oxides. In a specific embodiment, the gate electrode 305B iscomposed of a non-workfunction-setting fill material formed above ametal workfunction-setting layer. In an embodiment, the gate electrode305B is composed of a P-type material. The gate stack 305 may alsoinclude dielectric spacers 318, as depicted in FIG. 3A.

As shown in FIG. 3B, the source and drain regions 322 are “embedded,” or“raised” replacement source and drain regions. FIG. 3A furtherillustrates a cross-sectional view during fabrication of the device 300.Referring to FIG. 3A, portions of the Ge-rich device layer 308A and, inthe exemplary embodiment, portions of the top transition layer 307C, andeven portions of the p-type transition layer 307B are removed to providerecessed regions 320, on either side of the gate stack 305. Recessedregions 320 may be formed by any suitable technique that removesportions of the device layer 308A etc., such as a dry etch or a wet etchprocess. In one embodiment, at least a portion of the recessed regions320 are formed with a wet etch sensitive to the oxidation state of theGe-rich device layer 308A, such as but not limited to aqueous hydroxidechemistries like ammonium hydroxide (NH₄OH), potassium hydroxide (KOH),tetramethylammonium hydroxide (TMAH), or other tetraalkylammoniumhydroxides. In one embodiment, a first recessing of the regions 320entails a dry plasma etch to define the Ge-rich device layer 308A usingNF₃, HBr, SF₆, or Cl₂, while a second recessing of the regions 320entails a wet etch of the SiGe transition layer 307C using an aqueoushydroxide chemistry. The presence of the p-type transition layer 307B isadvantageous in formation of recessed regions 320, for example allowinga first etch of the Ge-rich device layer 308A, followed by a SiGe etchof the top transition layer 307C that is highly selective (nearinfinite) to the Ge-rich device layer 308A (being, for example pure Ge).As such, tips of the source, drain regions proximate to the ends of thedevice channel may be formed with great precision allowing for scalingof the gate length of the FET (L_(g)). In one embodiment, gate stack 305guides the formation of recessed regions 320 (i.e., self-alignedrecessed regions 320). In one embodiment, recessed regions 320 areformed with rounded corners, as shown in FIG. 3B. In another embodiment,however, recessed regions 320 are formed with faceted corners. In anembodiment, the n-type leakage suppression layer 307A serves as an etchstop during formation of the recessed regions 320. Referring again toFIG. 3B, a pair of embedded source, drain regions 322 is formed in therecessed regions 320, epitaxially, or not. In an embodiment, the source,drain regions 322 is uniaxially compressively stressing the Ge-richdevice layer 308 and is composed of a material having a lattice constantlarger than germanium, such as III-V materials having a lattice constantlarger than germanium.

As mentioned above, embodiments of the present invention may be appliedto non-planar MOS-FETs. For example, devices with a three-dimensionalarchitecture, such as tri-gate devices, may benefit from thesemiconductor device stack including a buried p-type transition layer.FIGS. 4A-4C illustrate angled views representing various operations in amethod of fabricating non-planar semiconductor devices employing thesemiconductor stack of FIG. 1A, in accordance with embodiments of thepresent invention.

Generally, non-planar FET embodiments can benefit from a buried p-typeGe etch suppression layer by enabling a fin to be formed in the Ge-richdevice layer that is of very fine lateral dimension (e.g., <22 nm).Subsequent to forming the Ge-rich fin surrounding and/or underlyingregions of SiGe may be etched with essentially infinite selectivity overthe Ge-rich fin structure such that channel lengths, and/or source/draintips, and/or sub-channel feature dimensions may be well-controlled withchemically sharp interfaces formed along the Ge-rich structures.

Referring to FIG. 4A, a semiconductor device 400 includes a gate stack405 disposed above a substrate 404. A three-dimensional Ge-rich devicebody 408A is disposed above the substrate 404 and underneath the gatestack 405. An isolation region 420 is disposed above the substrate 404with the three-dimensional device body 408A extending there from (devicebodies planar with isolation region 420 are also possible). A toptransition layer 407C is disposed above the substrate 404, below thethree-dimensional germanium-rich device body 408A. A Ge etch suppressionlayer 407B is disposed below the top transition layer 407C and has atleast some of the properties described elsewhere herein in the contextof the layer 107B of the device stack 100 (FIG. 1). The layer 407B isdisposed above a junction leakage suppression layer 407C that isdisposed above the buffer 406 and the substrate 404. Materialcompositions and dimensions of all the semiconductor layers in thedevice 400 are the same, or similar to, those described forsemiconductor device 300 as both device embodiments employ thesemiconductor stack 100.

In one embodiment, at least a portion of the isolation region 420 and/orthe non-planar semiconductor device stack surrounded by the isolationregion 420 is formed with a wet etch sensitive to the oxidation state ofthe Ge-rich device layer 408A, such as, but not limited to, the aqueoushydroxide chemistries described elsewhere herein (TMAH, etc.). In oneembodiment, a first etching of a semiconductor device stack (e.g., stack100) to form the isolation region 420 entails a dry plasma etch, suchas, but not limited to NF₃, HBr, SF₆, or Cl₂. A second etching, forexample of the thy-etch sidewall of the semiconductor stack exposed bythe dry etching, entails a wet etch of the SiGe transition layer 307Cusing aqueous hydroxide chemistries. Alternatively, or additionally, arecessing of the isolation region 420 relative to the non-planarsemiconductor body may include a wet etch sensitive to the oxidationstate of the Ge-rich device layer 408A, such as, but not limited to, theaqueous hydroxide chemistries described elsewhere herein (TMAH, etc.).In either situation, the presence of the p-type transition layer 307Benables a SiGe etch of the top transition layer 307C that is highlyselective (near infinite) to the Ge-rich device layer 308A (being, forexample pure Ge).

Portions of the device body 408A not disposed under the gate stack 405(and surrounding dielectric spacers 418) are doped source and drainregions. In accordance with an embodiment of the present invention, theisolation region 420 is recessed to the interface of thethree-dimensional Ge-rich (e.g., pure Ge) device body 408A and the toptransition layer 407C (e.g., Si_(0.3)Ge_(0.7)), as depicted in FIG. 4A.However, other embodiments may include setting the height of theisolation regions 420 above or below this particular interface.

In an embodiment, the source and drain regions are formed by doping(e.g., p-type) portions of the three-dimensional germanium active body406 uncovered by the gate stack 405. Portions of the transition layer407C may also be p-type doped in the source and drain regions withp-type dopants extending all through way into the p-type layer 407B toform p+/n diodes with the n-type leakage suppression layer 407A atopposite ends of the non-planar device body. However, in anotherembodiment, the source and drain regions are embedded source and drainregions. For example, FIGS. 4B and 4C illustrate cross-sectional viewsin the fabrication of another semiconductor device having a Ge-richdevice layer with an underlying diffusion barrier layer, in accordancewith another embodiment of the present invention.

Referring to FIG. 4B, portions of the three-dimensional Ge-rich body408A and, possibly, portions of the transition layer 407C and p-typetransition layer 407B are removed to provide recessed regions 422, oneither side of the gate stack 405. Recessed regions 422 may be formed byany suitable technique that removes portions of the three-dimensionalGe-rich device body 408A etc., such as a dry etch or a wet etch process.In one embodiment, at least a portion of the recessed regions 422 areformed with a wet etch sensitive to the oxidation state of the Ge-richdevice layer 408A, the aqueous hydroxide chemistries described elsewhereherein (TMAH, etc.). In one embodiment, a first recessing of the regions422 entails a dry plasma etch to define the Ge-rich device layer 408Ausing NF₃, HBr, SF₆ or Cl₂, while a second recessing of the regions 422entails the wet etch using NH₄OH or TMAH, or similar. The presence ofthe p-type transition layer 407B is advantageous in formation ofrecessed regions 422, for example allowing a first etch of the Ge-richdevice layer 408A, followed by a SiGe etch of the transition layer 407Cthat is highly selective (near infinite) to the Ge-rich device layer408A (being, for example, pure Ge). As such, tips of the source, drainregions may be formed with great precision allowing for scaling of thegate length of the FET (L_(g)). In one embodiment, gate stack 405 guidesthe formation of recessed regions 422, forming self-aligned recessedregions 422. In an embodiment, the n-type leakage suppression layer 407Aserves as an etch stop during formation of the recessed regions 422.

Referring to FIG. 4C, a pair of raised source, drain regions 424 isformed in the recessed regions 422, epitaxially, or not. In anembodiment, the source, drain regions 424 are uniaxially compressivelystressing the Ge-rich device layer 408A and are composed of a materialhaving a lattice constant larger than germanium, such as III-V materialshaving a lattice constant larger than germanium.

FIG. 5A illustrates an isometric sectional view of a nanowire ornanoribbon semiconductor device employing the semiconductor stack ofFIG. 1A, in accordance with an embodiment of the present invention. FIG.5B illustrates a cross-sectional channel view of the nanowire-basedsemiconductor structure of FIG. 5A, in accordance with an embodiment ofthe present invention. FIG. 5C illustrates a cross-sectional view of thenanowire-based semiconductor structure of FIG. 5A, in accordance with anembodiment of the present invention.

Referring first to FIG. 5A, a semiconductor device 500 includes one ormore vertically aligned or stacked germanium nanowires (508 set)disposed above a substrate 504. Embodiments herein include either singlewire devices or multiple wire devices. As an example, a twonanowire-based device having nanowires 508A, 508B is shown forillustrative purposes. For convenience of description, nanowire 508A isused as an example where description is focused on only one of thenanowires in the set 508. It is to be understood that where attributesof one nanowire are described, embodiments based on a plurality ofnanowires may have the same attributes for each of the nanowires.

Each of the Ge-rich (e.g., pure Ge) nanowires 508 includes a channelregion 506 disposed in the nanowire. The channel region 506 has a length(L). Referring to FIG. 5B, the channel region also has a perimeterorthogonal to the length (L). Referring to both FIGS. 5A and 5B, a gatestack 505 surrounds the entire perimeter of each of the channel regions506. The gate stack 505 includes a gate electrode along with a gatedielectric layer disposed between the channel region 506 and the gateelectrode (not individually shown). The channel region 506 is discretein that it is completely surrounded by the gate stack 505 without anyintervening material such as underlying substrate material (such as thetransition layer 107C in reference to the stack 100) or othersacrificial channel fabrication materials spacing apart the Ge-richnanowires 508. Accordingly, in embodiments having a plurality ofnanowires 508, the channel regions 506 of the nanowires are alsodiscrete relative to one another, as depicted in FIG. 5B. A junctionleakage suppression layer 507A is disposed above the substrate 504,below the one or more germanium nanowires 508. The gate stack 505 isdisposed over the n-type leakage suppression layer 507A, and may be onthe SiGe transition layer 507C, as illustrated. Although not depicted,in an embodiment, a buffer may be disposed directly between thesubstrate 504 and the junction leakage suppression layer 507A,substantially as described in the context of device stack 100.

Referring again to FIG. 5A, each of the nanowires 508 also includessource and drain regions 510 and 512 disposed in the nanowire on eitherside of the channel region 506. The source and drain regions 510/512 aredisposed on the SiGe transition layer 507C, as illustrated. In anembodiment, the source and drain regions 510/512 are replaced source anddrain regions, e.g., at least a portion of the nanowires is removed andreplaced with a source/drain material region. However, in anotherembodiment, the source and drain regions 510/512 are composed ofportions of the one or more germanium nanowires 508 that are merelydoped (e.g., by boron implant, etc.).

A pair of contacts 514 (dash lined in FIG. 5A) is disposed over thesource/drain regions 510/512. In an embodiment, the semiconductor device500 further includes a pair of spacers 516 (dash lined in FIG. 5A). Thespacers 516 are disposed between the gate stack 505 and the pair ofcontacts 514. As described above, the channel regions and thesource/drain regions are, in at least several embodiments, made to bediscrete. However, not all regions of the nanowires 508 need bediscrete. For example, referring to FIG. 5C, nanowires 508A-508B are notdiscrete at the location under spacers 516. In one embodiment, the stackof nanowires 508A-508B have intervening sacrificial semiconductormaterial there between (509B), and below (509A), which may be SiGe(e.g., of a lower Ge concentration than that of transition layer 107C),or silicon. In one embodiment, the bottom nanowire 508A is still incontact with a portion of the transition layer 507C, e.g., used infabrication as described below.

In an embodiment, the one or more Ge-rich nanowires 508 are composedessentially of germanium, the transition layer 507C is Si_(0.3)Ge_(0.7),the p-type Ge etch suppression layer 507B is p-type dopedSi_(0.3)Ge_(0.7) and the junction leakage suppression layer 507A isn-type doped Si_(0.3)Ge_(0.7), as described elsewhere herein for thedevice stack 100. In an embodiment, the one or more germanium nanowires508 are compressively stressed (e.g., by 1-2% relative to the transitionlayer 507C).

Although the device 500 described above is for a single device, e.g., aPMOS device, a CMOS architecture may also be formed to include both NMOSand PMOS nanowire-based devices disposed on or above the same substrate.In an embodiment, the nanowires 508 may be sized as wires with z and ydimensions substantially the same, or as ribbons with one of the z and ydimensions greater than the other. The nanowires 508 may havesquared-off, rounded, or faceted (e.g. at some angle non-orthogonal to zand y axis). Material compositions and dimensions may be the same orsimilar as those described for semiconductor stack 100, and device 300or 400.

In another aspect, methods of fabricating a nanowire semiconductorstructure are provided. For example, FIGS. 6A-6D illustratethree-dimensional cross-sectional views representing various operationsin a method of fabricating a nanowire semiconductor device having, atleast at one point in the process, a Ge-rich device layer with anunderlying SiGe transition layer, and a p-type doped Ge etch suppressionlayer, in accordance with an embodiment of the present invention.

Referring to FIG. 6A, a fm-type structure 612 is formed above asubstrate 604. The fin includes Ge-rich device layers 608A′ and 608B′and two intervening silicon-rich material layers 609A′ and 609B′, suchas a silicon or silicon germanium layers of high Si content than thedevice layer 608A′ and 608B′. The fin stops on the transition layer607C, although in other embodiments the fin-type structure 612 mayextend down to include a patterned portion of a transition layer 607C.Although not depicted, in an embodiment, a buffer is disposed directlybetween the substrate 604 and the junction leakage suppression layer607C.

FIG. 6B illustrates the fin-type structure 612 with three sacrificialgate structures 614A, 614B, and 614C disposed thereon. In one suchembodiment, the three sacrificial gates 614A, 614B, and 614C arecomposed of a sacrificial gate oxide layer 616 and a sacrificialpolysilicon gate layer 618 which are, e.g., blanket deposited andpatterned with a plasma etch process conventional to the art.

Following patterning to form the three sacrificial gates 614A, 614B, and614C, spacers may be formed on the sidewalls of the three sacrificialgates 614A, 614B, and 614C, and doping may be performed in regions 620of the fin-type structure 612 shown in FIG. 6B (e.g., tip and/or sourceand drain type doping), and an interlayer dielectric layer may be formedto cover and then re-expose the three sacrificial gates 614A, 614B, and614C. The interlayer dielectric layer may then be polished to expose thethree sacrificial gates 614A, 614B, and 614C for a replacement gate, ora gate-last, process. Referring to FIG. 6C, the three sacrificial gates614A, 614B, and 614C are exposed, along with spacers 622 and interlayerdielectric layer 624.

The sacrificial gates 614A, 614B, and 614C are then removed, e.g., in areplacement gate or gate-last process flow convention in the art for thematerials chosen, to expose channel portions of the fm-type structure612. Referring to FIG. 6D, the sacrificial gates 614A, 614B, and 614Care removed to provide trenches 626 and, thus, reveal channel portionsof the nanowires. Portions of the intervening sacrificial layers exposedby the trenches 626 are removed to leave discrete portions of theGe-rich device layers 608A′ and 608B′ to form nanowires 608A and 608B.In FIG. 6D, sacrificial material 609A is illustrated for clarity, butwould typically be removed concurrently with a sacrificial layerdisposed between 608A and 608B.

In an embodiment, the silicon-rich sacrificial layers 609A and 609B areetched selectively with a wet etch that does not etch the Ge-rich devicelayers 608A′ and 608B′ to release, or undercut, lengths of the devicelayers 608A′ and 608B′ not anchored by other structures (e.g., spacers622). In one embodiment, the wet etch is sensitive to the oxidationstate of the Ge-rich device layers 608A′ and 608B′. Etch chemistriessuch as, but not limited to aqueous hydroxide chemistries, includingNH₄OH, KOH, and TMAH, for example, may be utilized to selectively etchthe sacrificial layers 609A and 609B. The presence of the p-typetransition layer 607B is advantageous in improving the selectivity of aSiGe etch relative to the Ge-rich device layers 608A′ and 608B′. Inembodiments where the device layers 608A′ and 608B′ are example pure Ge,etch selectivity is nearly infinite to the nanowires, such that thesacrificial layers 609A and 609B may be removed along a chemically sharpinterface with the device layers 608A′ and 608B′(i.e., no portion of thedevice layers are etched).

In alternative embodiments, although not shown, the transition layers607C and 607B may also be removed, e.g., prior to, following, or at thesame time as removal of sacrificial layers 609A and 609B. Also, thediffusion barrier layer may be totally removed or only partiallyremoved, e.g., leaving remnants under the spacers, or alternatively maybe left intact. Subsequently device fabrication may be completed. In oneembodiment, a surrounding gate electrode is formed around the germaniumnanowires 604 and 608 and over the leakage suppression layer 507A, asdescribed above in association with FIG. 5A.

At the process stage depicted in FIG. 6D, channel engineering or tuningmay be performed. For example, in one embodiment, the discrete portionsof the Ge-rich device layer 608A and 608B may be thinned using oxidationand etch processes, etc. Accordingly, the initial wires formed fromGe-rich layers 608A′ and 608B′ may begin thicker and are thinned to asize suitable for a channel region in a nanowire device, independentfrom the sizing of the source and drain regions of the device.

Following formation of the discrete channel regions as depicted in FIG.6D, high-k gate dielectric and metal gate processing may be performedand source and drain contacts may be added. Contacts may be formed inthe place of the interlayer dielectric layer 624 portions remaining inFIG. 6D. Furthermore one or more thermal processes may anneal thesemiconductor layers such that the p-type doped layer 607B and n-typedoped layer 607A may diffuse together, even to the point that the p-typeand n-type dopants do not form separate peaks in a dopant profile asshown in FIG. 1B. The n-type doped layer 607A however is not completelycompensated by the p-type doped layer 607A and as the anneals may beperformed well after selective etching of the SiGe sacrificial layers,the function of the p-type doped layer 607A may still be realized.

FIG. 8 illustrates a computing device 700 in accordance with oneimplementation of the invention. The computing device 700 houses a board702. The board 702 may include a number of components, including but notlimited to a processor 704 and at least one communication chip 706. Theprocessor 704 is physically and electrically coupled to the board 702.In some implementations the at least one communication chip 706 is alsophysically and electrically coupled to the board 702. In furtherimplementations, the communication chip 706 is part of the processor704.

Depending on its applications, computing device 700 may include othercomponents that may or may not be physically and electrically coupled tothe board 702. These other components include, but are not limited to,volatile memory (e.g., DRAM), non-volatile memory (e.g., ROM), flashmemory, a graphics processor, a digital signal processor, a cryptoprocessor, a chipset, an antenna, a display, a touchscreen display, atouchscreen controller, a battery, an audio codec, a video codec, apower amplifier, a global positioning system (GPS) device, a compass, anaccelerometer, a gyroscope, a speaker, a camera, and a mass storagedevice (such as hard disk drive, compact disk (CD), digital versatiledisk (DVD), and so forth).

The communication chip 706 enables wireless communications for thetransfer of data to and from the computing device 700. The term“wireless” and its derivatives may be used to describe circuits,devices, systems, methods, techniques, communications channels, etc.,that may communicate data through the use of modulated electromagneticradiation through a non-solid medium. The term does not imply that theassociated devices do not contain any wires, although in someembodiments they might not. The communication chip 706 may implement anyof a number of wireless standards or protocols, including but notlimited to Wi-Fi (IEEE 802.11 family), WiMAX (IEEE 802.16 family), IEEE802.20, long term evolution (LTE), Ev-DO, HSPA+, HSDPA+, HSUPA+, EDGE,GSM, GPRS, CDMA, TDMA, DECT, Bluetooth, derivatives thereof, as well asany other wireless protocols that are designated as 3G, 4G, 5G, andbeyond. The computing device 700 may include a plurality ofcommunication chips 706. For instance, a first communication chip 706may be dedicated to shorter range wireless communications such as Wi-Fiand Bluetooth and a second communication chip 706 may be dedicated tolonger range wireless communications such as GPS, EDGE, GPRS, CDMA,WiMAX, LTE, Ev-DO, and others.

The processor 704 of the computing device 700 includes an integratedcircuit die packaged within the processor 704. In some embodiments ofthe invention, the integrated circuit die of the processor includes oneor more devices, such as MOS-FETs built in accordance with embodimentsdescribed elsewhere herein. The term “processor” may refer to any deviceor portion of a device that processes electronic data from registersand/or memory to transform that electronic data into other electronicdata that may be stored in registers and/or memory.

The communication chip 706 also includes an integrated circuit diepackaged within the communication chip 706. In accordance with anotherembodiment of the invention, the integrated circuit die of thecommunication chip includes one or more devices, such as MOS-FETs withfeatures and/or fabricated in accordance with embodiments describedelsewhere herein.

In further implementations, another component housed within thecomputing device 700 may contain an integrated circuit die that includesone or more devices, such as MOS-FETs with features and/or fabricated inaccordance with embodiments described elsewhere herein.

In embodiments, the computing device 700 may be a laptop, a netbook, anotebook, an ultrabook, a smartphone, a tablet, a personal digitalassistant (PDA), an ultra mobile PC, a mobile phone, a desktop computer,a server, a printer, a scanner, a monitor, a set-top box, anentertainment control unit, a digital camera, a portable music player,or a digital video recorder.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Furthermore, many other embodimentswill be apparent to those of skill in the art upon reading andunderstanding the above description. Although the present invention hasbeen described with reference to specific exemplary embodiments, it willbe recognized that the invention is not limited to the embodimentsdescribed, but can be practiced with modification and alteration withinthe spirit and scope of the appended claims. The scope of the inventionshould, therefore, be determined with reference to the appended claims,along with the full scope of equivalents to which such claims areentitled.

What is claimed is:
 1. A semiconductor device stack, comprising: agermanium (Ge)-rich device layer disposed above a silicon (Si)substrate; a SiGe transition layer disposed above the substrate, andbelow the device layer; a p-type δ-doped SiGe layer disposed above thesubstrate, and below the transition layer; an n-type SiGe layer disposedabove the substrate, and below the p-type δ-doped SiGe layer; and one ormore SiGe buffer layer disposed above the substrate, and below then-type SiGe layer.
 2. The semiconductor device stack of claim 1, whereinthe device layer is one of a plurality of device layers separated byintervening sacrificial semiconductor layers with a first device layerdisposed directly on a first sacrificial layer, the first sacrificiallayer disposed directly on the transition layer, the transition layer isdisposed directly on the p-type δ-doped SiGe layer, and the p-typeδ-doped SiGe layer is separated from the n-type SiGe layer by a SiGespacer layer.
 3. The semiconductor device stack of claim 2, wherein thesacrificial layers have a lower concentration of Ge than the devicelayers; and wherein the transition layer, the p-type δ-doped SiGe layer,the SiGe spacer layer, and the n-type SiGe layer all have a same siliconand germanium content.
 4. The semiconductor device stack of claim 3,wherein the transition layer is between 25 and 100 nm thick, the p-typeδ-doped SiGe layer is between 5 and 15 nm thick, the SiGe spacer layeris between 2 and 5 nm thick, and the n-type SiGe layer is between 5 and20 nm thick.
 5. The semiconductor device stack of claim 4, wherein thetransition layer is intrinsic, wherein the p-type δ-doped SiGe layer hasa boron dopant concentration of at least 1e18 cm⁻³, and wherein then-type SiGe layer has a phosphorus dopant concentration of at least 1e18cm⁻³.
 6. The semiconductor device stack of claim 4, wherein the devicelayers consist essentially of germanium, are undoped, and each has athickness between 5 and 15 nm, and wherein the sacrificial semiconductorlayers are composed of SiGe and each has a thickness between 5 and 30nm.
 7. The semiconductor device stack of claim 1, wherein at least thedevice layer and the transition layer are embedded within an adjacentisolation region also disposed the substrate.
 8. A semiconductor device,comprising: a gate stack including a gate dielectric and a gateelectrode disposed over the semiconductor device stack of claim 1,wherein the gate dielectric is in direct contact with the device layer;and a source region and a drain region disposed on opposite sides of thegate stack.
 9. The semiconductor device of claim 8, wherein the devicelayer comprises a non-planar body having two opposite sidewallsextending from a dielectric isolation region disposed above thesubstrate and adjacent to the device stack, and wherein the gate stackis disposed on the sidewalls.
 10. A semiconductor device, comprising: aplurality of germanium (Ge)-rich nanowires aligned vertically over asilicon (Si) substrate; a SiGe transition layer disposed between theplurality of nanowires and the substrate; a p-type δ-doped SiGe layerdisposed below the SiGe transition layer; an n-type doped SiGe layerdisposed between the p-type δ-doped SiGe layer and the substrate; a gatestack disposed over the plurality of nanowires and completelysurrounding a length of each of the plurality of nanowires; spacersdisposed adjacent the gate stack; and source/drain regions disposedadjacent to the spacers on opposite sides of the gate stack, in contactwith the plurality of nanowires.
 11. The semiconductor device of claim10, wherein the plurality of nanowires are vertically separated by anintervening sacrificial semiconductor layer along lengths of thenanowires covered by the spacers, with a first nanowire disposeddirectly on a first sacrificial semiconductor layer, the firstsacrificial semiconductor layer disposed directly on the transitionlayer, the transition layer disposed directly on the p-type δ-doped SiGelayer, and the p-type δ-doped SiGe layer separated from the n-type SiGelayer by a SiGe spacer layer.
 12. The semiconductor device of claim 11,wherein the germanium (Ge)-rich nanowires consist essentially of Ge,wherein the sacrificial semiconductor layer contains silicon.
 13. Thesemiconductor device of claim 12, wherein the transition layer isbetween 25 and 100 nm thick, the p-type δ-doped SiGe layer is between 5and 15 nm thick, the SiGe spacer layer is between 2 and 5 nm thick, andthe n-type SiGe layer is between 5 and 20 nm thick.
 14. Thesemiconductor device 13, wherein the transition layer is intrinsic,wherein the p-type δ-doped SiGe layer has a boron dopant concentrationof at least 1e18 cm⁻³, and wherein the n-type SiGe layer has aphosphorus dopant concentration of at least 1e18 cm⁻³.
 15. Thesemiconductor device of claim 13, wherein the nanowires along the lengthsurrounded by the gate stack are undoped and each has a thicknessbetween 5 and 15 nm, and wherein the sacrificial layers each have athickness between 5 and 30 nm.
 16. The semiconductor device of claim 10,wherein the gate stack is disposed directly on the transition layer. 17.A method of fabricating a nanowire semiconductor device, the methodcomprising: receiving a semiconductor device stack comprising: agermanium (Ge)-rich device layer disposed on a sacrificial semiconductorlayer comprising more silicon (Si) than that in the device layer ap-type δ-doped SiGe layer buried below the sacrificial semiconductorlayer; etching at least a partial thickness of the sacrificialsemiconductor layer selectively to the device layer with a wet etchantto undercut the device layer and form a discrete channel region of ananowire; and forming a gate stack completely surrounding the discretechannel region of the nanowire.
 18. The method of claim 17, wherein thewet etchant is selected from the group consisting of: ammoniumhydroxide, potassium hydroxide, and tetramethylammonium hydroxide(TMAH).
 19. The method of claim 19, wherein the wet etchant forms achemically sharp interface between the device layer and the sacrificialsemiconductor layer.
 20. The method of claim 17, wherein the etching isperformed without exposing the p-type δ-doped SiGe layer to the wetetchant.